This invention relates to semiconductors, and particularly to transistors for power switching that utilize field plates.
Power semiconductor devices are used as switches or rectifiers in power electronic circuits, such as switch mode power supplies. Common power devices include the power diode, thyristor, power MOSFET and IGBT (insulated gate bipolar transistor). A power diode or MOSFET, for example, operates on principles similar to those of its low power counterparts, but is able to carry a larger amount of current and typically can support a larger reverse-bias voltage in the off state.
Structural changes are often made in power devices to accommodate the higher current density, higher power dissipation and/or higher reverse breakdown voltage required. The vast majority of discrete (i.e., non integrated) power devices are built using a vertical structure, whereas small-signal devices employ a lateral structure. With the vertical structure, the current rating of the device is proportional to its area, and the voltage blocking capability is achieved with the height of the die. In the vertical structure, one of the connections of the device is located on the bottom of the semiconductor.
High electron mobility transistors (HEMTs) are a common type of solid state transistor that is regularly fabricated from semiconductor materials such as Silicon (Si) or Gallium Arsenide (GaAs). One disadvantage of Si is that it has low electron mobility (600-1450 cm2/V-s), which produces a high source resistance that can degrade high performance gain [CRC Press, The Electrical Engineering Handbook, Second Edition, Dorf, p. 994, (1997)].
GaAs based HEMTs have become the standard for signal amplification in civil and military radar, handset cellular, and satellite communications. GaAs has a higher electron mobility (approximately 6000 cm2/V-s) and a lower source resistance than Si, which allows GaAs based devices to function at higher frequencies. However, GaAs, like silicon, has a relatively small bandgap (1.12 eV for silicon and 1.42 eV for GaAs at room temperature) and relatively small breakdown voltage, which prevents GaAs and silicon based HEMTs from providing high power at high frequencies.
In response to these disadvantages with the Si and GaAs materials systems for high power applications, a major breakthrough in power semiconductor devices was achieved with the replacement of silicon by wide bandgap semiconductor, such as silicon carbide (SiC) and the Group III nitrides, e.g., gallium nitride (GaN). These materials typically exhibit higher electric field breakdown strength and higher electron saturation velocity as compared to GaAs and Si. Silicon carbide MOSFETS, for example, deliver 10 to 100 times better performance (or smaller size) than equivalent silicon based devices. SiC Schottky diodes with a breakdown voltage of 1200V are commercially available. As both are majority carrier devices, they can operate at high speed. Bipolar devices are being developed for higher voltages, up to 20 kV. Among its advantages, silicon carbide can operate at higher temperature (up to 400° C.) and has a lower thermal resistance than silicon, allowing better cooling.
In particular, GaN power HEMTs not only exhibit higher efficiency than both SiC and Si MOSFETs, but also perform well at higher frequencies, where Si simply does not function. GaN has the highest figure of merit of any semiconductor device for power switching. GaN HEMTs, owing to their high electron mobility and high breakdown field, exhibit a Baliga DC figure of merit for high voltage power devices which is superior to all other available semiconductors, resulting in ultra-low on resistance and a compact die size.
HEMTs can offer operational advantages in many circumstances because a two dimensional electron gas (2DEG) is formed in the HEMT structure at the heterojunction of two semiconductor materials with different bandgap energies, where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped, smaller bandgap material, and can contain a very high sheet electron concentration, in excess of, for example, 1×1013 carriers/cm2. In addition, electrons originating in the wider bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering. The combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for high frequency applications.
Innovations in GaN HEMT device technology have increased the breakdown voltage as well as the power performance for devices operable at RF and microwave frequencies greater than 0.5 GHz. High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to polarization in the AlGaN. HEMTs in the GaN/AlGaN system have already been demonstrated. U.S. Pat. Nos. 5,192,987 and 5,296,395 describe AlGaN/GaN HEMT structures and methods of manufacture. U.S. Pat. No. 6,316,793, which is commonly assigned and is incorporated herein by reference, describes a HEMT device having a semi-insulating silicon carbide substrate, an aluminum nitride buffer layer on the substrate, an insulating gallium nitride layer on the buffer layer, an aluminum gallium nitride barrier layer on the gallium nitride layer, and a passivation layer on the aluminum gallium nitride active structure.
Improvements in the manufacture of wide bandgap semiconductor materials, such as AlGaN/GaN, have helped advance the development of AlGaN/GaN transistors, such as high electron mobility transistors (HEMTs), for high frequency, high temperature and high power applications. AlGaN/GaN has large bandgaps, as well as high peak and saturation electron velocity values [B. Gelmont, K. Kim and M. Shur, Monte Carlo Simulation of Electron Transport in Gallium Nitride, J. Appl. Phys. 74, (1993), pp. 1818-1821]. AlGaN/GaN HEMTs can also exhibit two dimensional electron gas (2DEG) layer sheet densities in excess of 1013/cm2 and relatively high electron mobility (up to 2019 cm2/Vs) [R. Gaska, et al., Electron Transport in AlGaN—GaN Heterostructures Grown on 6H-SiC Substrates, Appl. Phys. Lett. 72, (1998), pp. 707-709]. These characteristics allow AlGaN/GaN HEMTs to provide very high voltage and high power operation at RF, microwave and millimeter wave frequencies.
AlGaN/GaN HEMTs have been grown on sapphire substrates and have shown a power density of 4.6 W/mm and a total power of 7.6 W [Y. F. Wu et al., GaN-Based FETs for Microwave Power Amplification, IEICE Trans. Electron. E-82-C, (1999). pp. 1895-1905]. More recently, AlGaN/GaN HEMTs grown on SiC have shown a power density of 30 W/mm at 8 GHz [Y.-F. Wu, A. Saxler, M. Moore, R. P. Smith, S. Sheppard, P. M. Chavarkar, T. Wisleder, U. K. Mishra, and P. Parikh, “30-W/mm GaN HEMTs by Field Plate Optimization”, IEEE Electron Device Letters, Vol. 25, No. 3, pp. 117-119, March 2004.] and a total output power of 22.9 W at 9 GHz [M. Micovic, et al., AlGaN/GaN Heterojunction Field Effect Transistors Grown by Nitrogen Plasma Assisted Molecular Beam Epitaxy, IEEE Trans. Electron. Dev. 48, (2001), pp. 591-596].
U.S. Pat. No. 5,192,987 discloses GaN/AlGaN based HEMTs grown on a buffer and a substrate. Other AlGaN/GaN HEMTs and field effect transistors (FETs) have been described by Gaska, et al., High-Temperature Performance of AlGaN/GaN HFETs on SiC Substrates, IEEE Electron Device Letters, 18, (1997), pp. 492-494; and Wu, et al. “High Al-content AlGaN/GaN HEMTs With Very High Performance”, IEDM-1999 Digest, pp. 925-927, Washington D.C., December 1999. Some of these devices have shown a gain-bandwidth product (fT) as high as 100 gigahertz (Lu, et al. “AlGaN/GaN HEMTs on SiC With Over 100 GHz ft and Low Microwave Noise”, IEEE Transactions on Electron Devices, Vol. 48, No. 3, March 2001, pp. 581-585) and high power densities up to 10 W/mm at X-band (Wu et al., “Bias-dependent Performance of High-Power AlGaN/GaN HEMTs”, IEDM-2001, Washington D.C., Dec. 2-6, 2001) and Wu et al., High Al-Content AlGaN/GaN MOSFETs for Ultrahigh Performance, IEEE Electron Device Letters 19, (1998), pp. 50-53].
Electron trapping and the resulting differences between DC and RF characteristics have been a limiting factor in the performance of these devices. Silicon nitride (SiN) passivation has been successfully employed to alleviate this trapping problem, resulting in high performance devices with power densities over 10 W/mm at 10 Ghz. U.S. Pat. No. 6,586,781, for example, which is incorporated herein by reference in its entirety, discloses methods and structures for reducing the trapping effect in GaN-based transistors. Due to the high electric fields existing in these structures, however, charge trapping is still an issue.
Overlapping gate structures, or field plates, have been used to modify the electric field and thereby enhance the performance of GaN-based HEMTs at microwave frequencies. See Zhang et al., IEEE Electron Device Letters, Vol. 21, pp. 421-423 (September 2000). Karmalkar et al. performed simulations for the field plate structure, predicting up to five times enhancement in breakdown voltages. Karmalkar et al., IEEE Trans. Electron Devices, Vol. 48, pp. 1515-1521 (August 2001). Ando et al. used a similar structure with smaller gate dimensions and demonstrated performance of 10.3 W output power at 2 GHz using a 1 mm wide device on a SiC substrate. Ando et al., IEEE Electron Device Letters, Vol. 24, pp. 289-291 (May 2003). Chini et al. implemented a new variation of the field plate design with further reduced gate dimensions and obtained 12 W/mm at 4 GHz from a 150 μm-wide device on a sapphire substrate. Chini et al., IEEE Electron Device Letters, Vol. 25, No. 5, pp. 229-231 (May 2004). GaN based HEMTs with field plates have boosted power density to greater than 30 W/mm at frequencies up to 8 GHz. See, e.g., Y-F Wu et al, IEEE Electron Device Letters, Vol. 25, No. 3, pp. 117-119 (March 2004).
The approaches known in the art, however, have limitations for high performance applications in power switching. Consequently, a need has developed in the art for power switching devices which performance well at high frequencies, including higher breakdown voltages and lower on resistances.